1. Field of the Invention
This invention relates to a method and device for determining defects within a crystallographic substrate and more particularly to non-contact, non-destructive detection of defects within the substrate.
2. Description of the Relevant Art
Surface photovoltage (SPV) techniques for measuring the presence of heavy metal contaminants are well known. SPV detection of metal contaminants generally includes many steps. SPV utilizes light directed upon a semiconductor and, if the energy of the incident light (photons) is above the semiconductor bandgap, E.sub.g, then the incident photons are absorbed and produce excess carriers (holes and electrons). The concept of directing photons upon a substrate and producing minority carriers is often described as the recombination or "photogeneration" process. As minority carriers are photogenerated by the impinging photons, a certain number of the carriers reach the proximity of the substrate surface and become separated by the electric field of the surface space charge region to produce photovoltage.
The amount of minority carriers which reach the surface and are detected as photovoltage is directly proportional to the amount of recombination centers within a substrate. Many researchers have discovered a correlation between the presence of heavy metal contaminants and the introduction of recombination centers within the substrate bandgap. See, e.g., Lagowski, et al., "Non-Contact Mapping of Heavy Metal Contamination for Silicon IC Fabrication", (IOP Publishing, Ltd. 1992) pp. A185-A192 (herein incorporated by reference). Heavy metal introduction of recombination centers will increase the likelihood that minority carriers will diffuse to a lesser extent through a semiconductor during their lifetime. Accordingly, presence of heavy metal contaminants set forth a reduction in diffusion length L. Thus, SPV has been successfully used as a technique for quantifying heavy metal contaminants by determining fluctuations in L.
Measurement of diffusion length L begins by measurement of photovoltage and photon flux at several wavelengths of incident photon energies. The magnitude of the photovoltage is varied by adjustment of the incident light intensity or photoflux. Photovoltage at each light intensity value is plotted in relation to the reciprocal absorption coefficient, .varies..sup.-1. The plot is then linearly extrapolated to determine the intercept point along the reciprocal coefficient axis to obtain the minority carrier diffusion length L. L corresponds to the distance along the reciprocal coefficient axis at points in which the reciprocal absorption coefficient are negative. A general outline of the technique for measuring diffusion length L and correlating diffusion length L to the presence of heavy metal contaminants is described in U.S. Pat. No. 5,025,145 (herein incorporated by reference).
The determination of diffusion length and the correlation of diffusion length to heavy metal contaminants is well founded in the principle of defining reciprocal photovoltage as a linear function to reciprocal absorption coefficient. However, it is well known that reciprocal photovoltage is not always a linear (monotonically increasing) function of reciprocal absorption coefficient. Several points in the plot of reciprocal photovoltage versus reciprocal absorption coefficient may not satisfy the monotonically increasing criteria. In such case, points which do not meet the monotonically increasing criteria are rejected from the computation of the diffusion length. Only those points which are linear are utilized for extrapolation and computation of diffusion length L.
While determination of diffusion length L is well suited for detecting metal impurities, the procedure is often burdensome. First, the procedure requires the rejection of non-linear points from the computation model. It is oftentimes difficult to determine which points must be rejected (i.e., which points fall outside the monotonically increasing criteria). If certain "valid" points are rejected and certain points which should be "invalid" are accepted, then the extrapolated diffusion length L will be inaccurate. Thus, the data processor which accumulates photovoltages and performs the criteria modeling must be well attuned to accept only valid points while rejecting all invalid points. Secondly, the procedure requires a full mapping of each point in order to extrapolate valid points along a straight line to an x-axis intersect point (representative of diffusion length L). The x-intercept methodology and full extrapolation technique can be time consuming.
Although diffusion length L has been well studied and is generally accepted in the industry as an important application of SPV in the non-destructive detection of heavy metal contaminants, other techniques which utilize SPV for measuring other properties of the substrate have not been properly studied and applied to semiconductor manufacture. Specifically, during fabrication upon and into a crystallographic substrate, damage can occur at various points within the substrate. The damage can be present within the initial starting material or can occur during subsequent fabrication steps and present itself as defects within the crystallographic substrate. As defined herein, crystallographic refers to single crystal material such as single crystal silicon. As further defined herein, "defects" refers to any non-uniform material or structure present within a crystallographic (single crystal) uniform substrate. The defect material or structure can be present in the initial substrate starting material or can arise from ion implantation. Exemplary defects include areas of lattice dislocation caused, for example, by (i) an excessive concentration of diffusant, (ii) slippage caused by thermally induced stress, or (iii) ion implantation.
Anneal may not always remove implant-induced defects. Thus, it is important to be able to detect the occurrence of such defects at each of the processing steps. By using a non-contact, non-destructive method, the wafer can be maintained throughout the processing steps and defects can be charted in order to determine their source. Additionally, it would be advantageous to determine the relative success of anneal in removing crystallographic defects caused by ion implantation such that, if necessary, additional anneal steps at possibly higher temperatures can be utilized.
It would be further advantageous to provide a method for determining crystallographic defects using the advantages of SPV techniques but without having to incur the burdensome disadvantages of determining diffusion length L. The problems of determining non-linear points, rejecting non-linear points, and extrapolating diffusion length are to be avoided if the methodology for determining crystallographic defects is to be both efficient and rapid.